The present invention relates to a device and a method for locating one or more mobile elements in a predetermined zone. A particularly useful application is measurement of the activity of several animals (rodents, etc.) in a cage. However, the present invention has a wider scope, as it can be applied in various fields. There may be mentioned in particular:                Locating parts of the human body, for example the fingers of a hand for producing a sculpture, modelling of shapes with the fingers, man-machine interface for computer applications, etc.,        Locating objects with respect to a surface (pieces for a board game, etc.),        Interfaces of graphics tablets, control devices, etc.        etc.        
Generally, in the area of animal experiments, it is sometimes necessary to measure the movements of several animals (rodents, etc.) in a cage. These animals may be mice, rats or others. Such rodents are placed in a cage connected to a device for measuring their movements. Such a device is intended for example to assess the treatment of diseases and to monitor motor performance. This device may be produced within the optical field, the mechanical field or the electrical field.
In the optical field, it is possible to use infrared or any camera in the visible region for monitoring the movement of each mouse. However, this has many drawbacks, such as the difficulty of correctly identifying more than two rodents, or the overall dimensions of the device, as the rodents must be “visible”, etc.
In the mechanical field, it is possible to use the dynamic weighing technique. The drawbacks are measurement of activity limited to a single rodent, and an excessive likelihood of interference from objects in the cage (sawdust, excrement, etc.). It is also possible to use the acoustic pulse recognition technique, although this has the drawbacks of inability to identify the rodents and incompatibility with the presence of sawdust.
In the electrical field, the most promising technique is certainly radiofrequency. For example, document U.S. Pat. No. 5,853,327, describing a games platform for locating pieces that can be moved on the platform, is known in the games field. Each piece comprises a coil and a transponder for transmitting a unique identifier in response to excitation originating from a coil placed in the platform. This platform comprises an array of excitation coils. The response originating from the piece is picked up by a winding arranged in the platform, separate from the excitation coils. The piece is supplied from a built-in battery, or from the excitation signal, in which case the piece comprises a resonance capacitor connected to the coil incorporated in the piece. The drawback of such a system is complex addressing of the excitation coils, which are excited one by one.